System for absorption of explosive energy by pressure mitigation

ABSTRACT

1. A system for absorbing excessive pressures resulting from explosive forces generated by exploding material comprising. (a) a container; (b) a housing disposed in said container and occupying a first volume; (c) a source of explosive material disposed in said housing; (d) a second unfilled volume within said container and exclusive of said first volume; (e) a compressible closed-cell material occupying a third volume within said container and exclusive of said first and second volumes, said third volume being interfaced with said second volume; (f) venting means in said housing for permitting transmission of said explosive material from first volume to said second volume.

Nov. 5, 1974 J. M. EVJEN ETAL SYSTEM FOR ABSORPTION OF EXPLOSIVE ENERGYBY PRESSURE MITIGATION 2 Sheets-Sheet 1 Filed May 50, L972 Nov. 5, 1974J. M. EVJEN ETAL SYSTEM FOR ABSORPTION OF EXPLOSIVE ENERGY BY PRESSUREMITIGATION 2 Sheets-Sheet :3.

Filed May 50, 1972 United States Patent 3,846,178 SYSTEM FOR ABSORPTIONOF EXPLOSIVE ENERGY BY PRESSURE MITIGATION John M. Evjen, Gainesville,and Jerry Herrin, Alachua, Fla., assignors to General Electric CompanyFiled May 30, 1972, Ser. No. 257,694 Int. Cl. H01m 1/04 US. Cl. 136-166Claims ABSTRACT OF THE DISCLOSURE Excessive pressures resulting fromenergy generated by exploding material such as gases in a container suchas, for example, a battery case, are absorbed by compressible materialssuch as low density closed cell materials. Preferably, the volume offree space within the container is kept as small as possible withrespect to the volume of the compressible material space both to reducethe volume in which the explosive materials may accumulate and toprovide a large ratio of expansion to provide maximum pressurereduction.

BACKGROUND OF THE INVENTION This invention relates to a system forabsorbing explosive energies. In one aspect, the invention relates to asystem for absorbing energies generated by exploding gases in a closedcontainer.

Explosive substances such as gases, liquids, or solids may sometimes begenerated and thereafter accumulate in closed devices such as a batterycontaining a plura'ity of cells. The substances may be gases, or liquidor solid explosive substances. These substances may be generated bychemical reaction of other materials within the enclosure. Explosion ofsuch material causes a sudden rise in pressure in the free space withinthe container which, if not in some manner contained or absorbed, willin turn burst containers not normally designed to withstand suchabnormal pressure. This is true not only of sealed devices but ventedcontainers as well where, for example, a pressure relief vent isintended to prevent gradual pressure buildup. The problem can also beencountered in a system containing purging means to prevent accumulationof explosive gases-even at atmospheric pressureswhere the purgingmechanism fails to purge the container for one reason or another. Whilecontainers can be made heavier in anticipation of such pressures, thiscan add considerable weight to the device which, in the case of anaircraft battery, is highly undesirable.

In one particular example a sealed battery case containing a number ofcells may accumulate a mixture of hydrogen and oxygen gases given off bythe cells during the course of charging or discharging. While pressurerelief valves may be employed in the casing to vent accumulated gaseswhich build up pressure, the gases, even at ambient pressure, may stillrepresent an explosive mixture which is capable, upon ignition, ofgenerating pressures many times the original ambient pressure.

Several solutions have been proposed including the construction of acontainer having suflicient strength to withstand such pressures.However, such a container would add to both the cost and weight of thedevice. It has also been suggested to employ breakaway sections in thecontainer in the manner of safety pressure relief type vents. However,small vent-like ports which would merely vent the gas may not reduce thepressure rapidly enough to prevent explosion of the entire containerwhile larger breakaway portions may still result in spewing out thecontents of the device. It has also been proposed to increase theflexibility of the container walls so that they may expand, thusincreasing the container volume to contain the gases resulting from theexplosion at more moderate pressures.

However, it would be desirable to contain any explosion with little orno change in the design of the container. It is therefore an object ofthe invention to provide a system of absorbing abnormal pressuresgenerated within a sealed device by the explosive energy generated byignition of explosive substances therein. It is another object of theinvention to provide means for absorbing abnormal pressures within adevice without substantially increasing the weight of the device. It isyet another object of the invention to provide means for absorbingabnormal pressures generated by explosions within a device withoutsubstantially increasing the volume of the device. These and otherobjects of the invention will be apparent from the drawings anddescription.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cutawayisometric view of a battery case containing the system of the invention.

FIG. 2 is a top sectional view of the case of FIG. 1.

FIG. 3 is a rear sectional view of the case of FIG. 1.

FIG. 4 is a partially exploded and cutaway isometric view of the batteryof FIG. 1.

DESCRIPTION OF THE INVENTION Referring now to the figures, the system ofthe invention is illustrated as applied to a battery comprisingnickelcadmium cells used, for example, in an aircraft. A battery isgenerally indicated at 2 comprising an outer casing 4 having a cover 6thereon. Battery 2 contains a plurality of cells 10 therein which areseries-connected together to provide a preselected voltage at terminals22 on the exterior of casing 4 of battery 2. Each of the cells isprovided with a vent 12 thereon which permits gases generated within thecell to vent into the casing at a given pressure. The casing may beprovided with conventional venting means a and 80b for mitigation ofpressure buildup due to these gases.

While the cells occupy most of the volume of the casing, a small volume40 on top of the cells is open to permit access to each of the cells andthe cell terminals for interconnection of the cells together. Thisspace, together With other spaces between and around the cells, etc.,and hereinafter called the free volume, while small with respect to theentire volume of the battery case, is sufficiently large to allowhydrogen and oxygen gases to accumulate therein. These gases areliberated by the cells on overcharge in the proper proportion to providean explosive mixture which can, upon ignition, raise the internalpressure to greater than p.s.i.a pressure much higher than casing 4 isnormally designed to withstand. It should be noted here that theignition and explosion of accumulated gases within a device is a veryabnormal 0ccurrence and does not occur with suflicient regularity as towarrant the added expense and weight of increasing the thickness of allbattery casings as this would add undesirable weight to the aircraft,thus decreasing the payload.

It should be noted here that the system of the invention does notnecessarily prevent such an explosion or prevent damage to the contentsof the container. However, the system of the invention will preventdamage to external surroundings in the unlikely event of such anexplosion by absorbing the generated energy within the container.

In accordance with the invention, however, a portion of the battery caseis filled with a compressible cellular material 50 which is preferably aclosed cell foam material. As best seen in FIG. 4 this material, in apreferred embodiment, comprises a series of foam blocks 52, 54, and 56which have been cut and shaped to fit respectively into the volumebetween the two rows of cells; the space in front of the short row ofcells under the leads to the external terminals 22; and in the cover,i.e., over the cells.

In accordance with the invention, a substantial portion of the freevolume within the battery not taken up by the cells, preferably as muchas possible, is occupied by the compressible cellular material toprovide a minimum volume of open space available for accumulation ofgases. The use of the term substantial portion is intended to define atleast by volume of the free volume and preferably over 50%. Uponexplosion of the gases, the gassuch as air-within the closed cells ofthe cornpressible cellular material is compressed by the pressure of theexpansion of the ignited gases until a pressure equilibrium is achieved.When the volume of space within the battery casing occupied by thecompressible cellular material is large with respect to the volume offree space in which the gases accumulate, the effect of Compression ofthe cellular material results in a large reduction in the pressure whichwould otherwise be developed by the explosion since the gases generatedby the explosion expand into a volume (due to compression of thecellular material) which is much larger than that of the initial volume.Thus, for example, the pressure developed Within casing 4 using thesystem of the invention could be reduced from 90-150 p.s.i. to about 50p.s.i. if the volume of compressible material was twice the volume offree space in casing 4 (not considering any expansion of the containerwalls and assuming compression of the cellular material With negligibleinertia and elastic effects).

While the foregoing refers specifically to a battery in which oxygen andhydrogen gases are present, the invention can be applied to otherdevices in which other explosive substances can form, accumulate, or bestored. Depending upon the types of gases or other materials which maybe generated or reacted together to form ignitable substances and thefree volume to which gases resulting from such an ignition will beexposed (before compression of the cellular material), the volume ofcompressible cellular material may be increased or decreased. That is,if the particular explosive substances known to exist within a givendevice are capable, Without expansion, of increasing the pressure, uponignition, to, for example, three times that of ambient pressure and thecontainer is only capable of withstanding pressures of, for example,twice ambient pressure, the volume of compressible cellularmaterial-before compressionneed only be approximately one-half thevolume of the unoccupied free space within the container or /3 of thetotal, otherwise unused, volume. After ignition, the pressure generatedwill be effectively lowered by /3 due to compression of the compressiblematerial-because the volume will have been increased by 50%. On theother hand, if the pressures capable of being generated are much larger,a larger ratio of volume of compressible material to volume of freespace will be necessary.

The compressible cellular material to be used within the sealedcontainer comprises closed cell, low density material such as foamedpolymeric materials such as, for example, foam polyethylene,polyurethane, or other easily compressible material. In a preferredembodiment, the closed cellular material may be made from a polymerwhich also contains fire retardant additives and which has been expandedby the use of gases which are themselves not easily ignitable. The foamsmay be made, for example, using carbon dioxide liberating blowing agentswell known to those skilled in the art which result in a foam structurecontaining carbon dioxide gases which are not as combustible as would befoams using other blowing agents.

The foamed structure may comprise small cell or large cell foams but ineither event should comprise a low density foam structure; that is, theratio of open space or gas-filled space to the volume of polymericmaterial should be high to provide a maximum volume of compressiblesubstance, i.e., gaseous rather than solid. While the invention thuscontemplates foamed plastics as the preferred compressible material, thecell size is not deemed to be important and may, indeed, comprise one ormore sealed cells such as sealed bags of air or inert gas. The uase ofthe term foam structure is therefore intended herein to define a closedcell structure comprising at least one cell.

While the invention has been described with respect to utilization ofsuch a system in a sealed aircraft battery containing cells which maygenerate hydrogen and oxygen, the use of such a system in other sealedcontainers is to be considered within the scope of the invention whichis to be limited only by the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A system for absorbing excessive pressures resulting from explosiveforces generated by exploding material comprising.

(a) a container;

(b) a housing disposed in said container and occupying a first volume;

(c) a source of explosive material disposed in said housing;

(d) a second unfilled volume within said container and exclusive of saidfirst volume;

(e) a compressible closed-cell material occupying a third volume withinsaid container and exclusive of said first and second volumes, saidthird volume being interfaced with said second volume;

(f) venting means in said housing for permitting trans mission of saidexplosive material from first volume to said second volume.

2. The system of claim 1 wherein said closed cell material comprises lowdensity foamed polymeric substances.

3. The system of claim 1 wherein said second volume is less than thevolume occupied by said compressible closed cell material.

4. An explosion-resistant rechargeable battery system comprising anenclosed casing having therein (a) a plurality of vented electrochemicalcells capable of generating explosive gas mixtures of hydrogen andoxygen;

(b) a free volume within said casing not occupied by said cells; and

(c) pressure absorption means comprising a compressible, low density,closed cell foam ocupying a substantial portion of said free volume,said explosive gas mixture tending to collect in the unfilled volume ofsaid free volume not containing said absorption means whereby upon anexplosion of said explosive gas mixture, the pressure generated by saidexploding gases in said unfilled volume will be redistributed into anincreased volume due to the compression of said foam.

5. The battery system of claim 4 wherein said foam occupies at least 50%of said free volume.

6. The battery system of claim 4 wherein said foam comprises foamedpolyethylene.

7. The battery system of claim 6 wherein said casing is further providedwith (a) terminal means adjacent said cells;

(b) a removable cover;

(c) a first layer of said foamed polyethylene Within said casingadjacent said terminal means; and

(d) a second layer of said foamed polyethylene with said casing adjacentsaid removable cover.

8. The battery system of claim 4 wherein said casing is further providedwith venting for mitigating pressure buildup.

9. An explosion-resistant rechargeable battery system comprising arectangular parallelepiped casting of about 30-40 mils thickness havinga removable cover thereon and having at least two parallel rows ofrechargeable vented nickel-cadmium cells therein separated by a firstlayer of foamed polyethylene, terminal means on said casing adjacent oneof said cell rows and a second layer of foamed polyethylene adjacentsaid terminal means, and a third layer of foamed polyethylene adjacentsaid removable cover, said vented cells capable of generating explosivemixtures of hydrogen and oxygen during overcharge, said three layers offoam occupying at least half of the free volume within said casing notoccupied by said cells said cells generating explosive gas mixtures ofhydrogen and oxygen during overcharge, said gas mixture tending tocollect in the unfilled volume of said free volume not filled by saidthree layers of foam whereby upon explosion of said gases, the energyreleased thereby will be absorbed within said casing and therebypreventing rupture of said casing.

10. The system of claim 4 wherein said explosive gas mixture, uponignition, is capable of generating a first pressure within a fixedvolume equal to the unfilled volume, said casing has a structuralstrength capable of withstanding without rupturing up to a secondpredetermined internal pressure, said first pressure is greater than orequal to said second pressure, and the magnitude of said ReferencesCited UNITED STATES PATENTS 3,597,279 8/1971 Branche 136173 2,812,37811/1957 Barrett 136l66 3,314,823 4/1967 Balaguer 136173 OTHER REFERENCESTeach: Polystyrene, Reinhold Plastics Applications Series, New York,Nov. 15, 1960, pp. 128 and 134.

DONALD L. WALTON, Primary Examiner Us. 01. X.R. l36179 UNITED STATESPATENT OFFICE v CERTIFICATE OF CORRECTION Patent No, 846, 178 DatedNovember 5, 1974 Inventor(s) John M. Evjen and Jerry Herrin It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 1, Column 4, Line 33, insert "said" between from and first Claim9, Column 4, Line 73, "casting" should be casing Signed and sealed this4th day of February 1975,

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents uscoMM-oc e037 e-pea FORM PO-1050 (10-69) w u.s. sovsnuusmPRINTING omc: lacs o-ase-su

1. A system for absorbing excessive pressures resulting from explosiveforces generated by exploding material comprising. (a) a container; (b)a housing disposed in said container and occupying a first volume; (c) asource of explosive material disposed in said housing; (d) a secondunfilled volume within said container and exclusive of said firstvolume; (e) a compressible closed-cell material occupying a third volumewithin said container and exclusive of said first and second volumes,said third volume being interfaced with said second volume; (f) ventingmeans in said housing for permitting transmission of said explosivematerial from first volume to said second volume.